I liked the focus on non nanite robots. It reminded me a lot of the latter parts of the Crystal Society trilogy. It's not something you see a lot of, usually these stories either have the AI manipulating people or ascending to godhood overnight (or both).

Narratively, the story felt a bit bleak throughout. Still, that'd very in character since the narrator is telling the story after having been trapped and given up.

Good science fiction. 

There are reasons that couldn't easily happen in reality. And that is that evolution is too slow and dumb. It took billions of years for evolution to produce intelligence the first time. Machines might be quicker, but not that much. Now could a more sophisticated process produce intelligence? Yes. But in that case, you don't have a plain replicator, you have a sophisticated AI algorithm studying how the last robot failed and designing the next. 

I would say that robots are less evolveable. A small mutation to a lifeform is more likely to be beneficial than a small change to a robot.